Obese kids have lower levels of vitamin D than their slimmer counterparts, regardless of season, sex, or race and ethnicity, researchers reported.

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Obese kids have lower levels of vitamin D than their slimmer counterparts, regardless of season, sex, or race and ethnicity.

The obese children were also more likely to show signs of insulin resistance.

Obese kids have lower levels of vitamin D than their slimmer counterparts, regardless of season, sex, or race and ethnicity, researchers reported.

In a cross-sectional study, 92% of obese children had inadequate levels of 25-hydroxyvitamin D (25[OH)]D), compared with 68% of slimmer kids, according to Michele Hutchison, MD, PhD, and colleagues at the University of Texas Southwestern Medical Center in Dallas.

And half of the obese children had vitamin D deficiency, compared with 22% of thinner kids, Hutchison and colleagues reported in the January issue of the Journal of Clinical Endocrinology & Metabolism.

The obese children were also more likely to show signs of insulin resistance, the researchers reported.

The study suggests that "vitamin D deficiency is common in children in this southern United States location, and is significantly more prevalent in obese children," the authors concluded.

The findings come from a snapshot of 411 children, ages 6 through 16, whose body mass index (BMI) was at least in the 95th percentile for age, and who had been referred to an obesity center at Children's Medical Center Dallas.

They were compared with 87 children -- matched to the obese group by season when the data was acquired, age, and race/ethnicity -- who were below the 85th percentile of BMI for age. They were a convenience sample of otherwise healthy patients being treated for congenital hypothyroidism, acquired primary hypothyroidism, or isolated growth hormone deficiency.

The researchers measured levels of 25(OH)D, blood sugar, and serum insulin, as well as BMI and blood pressure. Participants were also asked about their diet, including daily intake of soda, juice, and milk, average daily fruit and vegetable intake, and whether or not they routinely skipped breakfast.

The recent Institute of Medicine report, published after this research was completed, was stricter, defining inadequacy as 12 to 20 ng/mL and deficiency as below 12 ng/mL, the researchers noted. But even using those stricter guidelines, more of the obese children were deficient -- 12.7% versus 3.4%.

Analysis showed:

On average, the obese children had a serum 25(OH)D concentration of 49 nmol/L, compared with 67.5 nmol/L for the slimmer cohort. The difference was significant at P<0.0001

The difference remained significant – at levels from P=0.01 to P<0.0001, regardless of sex, season, and race or ethnicity

Greater intake of soda and juice, as well as routinely skipping breakfast intake, were associated with lower serum 25(OH)D, at P<0.001, P=0.009, and P<0.001, respectively

Low 25(OH)D levels were also correlated with worse homeostasis model assessment of insulin resistance (HOMA-IR) and two-hour glucose after an oral glucose tolerance test, at P=0.001 and P=0.04, respectively

"Our study found that obese children with lower vitamin D levels had higher degrees of insulin resistance," said lead author Micah Olson, MD, also of the University of Texas Southwestern Medical Center.

The authors said were not able to measure physical activity or sunlight exposure in either the obese or nonoverweight group, and comparison of this information would have been helpful in explaining the mechanism behind the difference.

In a statement, Olson said the study cannot prove a cause-and-effect relationship but "does suggest that low vitamin D levels may play a role in the development of type two diabetes."

More research is needed, he added, "to determine the clinical significance of lower vitamin D levels in obese children," as well as whether treatment with vitamin D can alter such things as insulin resistance."

The study was supported by the NIH. The journal said the authors did not declare any conflicts.

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